A couple of years ago I read about an interesting experiment. It revealed something I wish I had known in my college years and it explained a weird phenomenon that ruined my love life.

I grew up in the age of the "liberated woman." It was a time when women didn't wear bras, Helen Reddy was singing "I am woman, hear me roar!" and men were being encouraged to show their feminine, emotional side.

There was a lot of casual sex back then -- this was almost a decade before HIV and herpes -- and many women were exercising their "right" to postpone reproduction by taking birth control pills.

The love of my life was Jane. We were both hippies and lived in a co-op dorm at the University of Massachusetts. We shared everything -- a dorm room with one bed, several Beatle's albums and an old Saab. Life was good. My hair was long and I had declared myself a "pacifist" to avoid being shipped off to the Vietnam War. Jane had been taking "the pill" but she had decided to stop, hoping that we could start a family. Marriage was optional back then.

Then, something quite unexpected happened. After about 6 weeks, Jane didn't come home one night. She didn't have a very good excuse either. She repeated the act a couple more times. Then, quite abruptly and dramtically, she left me for an ex-Marine with short hair and a Sylvester Stallone physique!

This dude was my exact opposite. He was crude, verbally abusive and very aggressive. I once called him a bad name when he appeared with his arm around Jane and he quickly dispatched a punch that gave me a black eye.

I could never understand how this happened until I read about the experiment. First reported in Nature, the study was carried out by researchers in Scotland and Japan. They asked women to select the one face from a range of images that they were most attracted to as a partner for a short-term sexual relationship.

The faces were generated by a computer and showed a range of facial features that varied from very feminine to very masculine.

[Above:] The faces are computer averages: the left face is more feminine, the right more masculine.

The ressults of this experiment showed that women who are ovulating (i.e. fertile) tend to be attracted to so-called "manly men," those with more masculine facial features and traits of dominance and competitiveness. They also tend to prefer the man who is not like them, genetically speaking.

The scientific explanation is that men who look more masculine have higher levels of male hormones (like testosterone) and also show a better ability to fight off disease and other worldly threats. This makes them attractive as potential mates because they will not only protect the female against harm but their children will inherit these useful characteristics. The preference for genetically "opposite" characteristics is thought to promote bio-diversity, promoting a more varied immune system and a stronger gene pool. But there was more.

The choice of the man's face or the preference for his "opposite" genes did not vary for women using the pill (because they were not fertile). So it seems that my Jane, once free from the effects of her contraceptives, allowed nature to impose its values and my "emotional, feminine personality" routine failed miserably.

What does this mean?

The role that contraceptives have had in changing modern culture has only recently been assessed. While much was made, at the time, of the "liberating" effect this had on women, the effects of taking "the pill" on the human species is now becoming the focus of serious research.

The possible impact of "the pill"

Three facts are worthy of consideration: First, the dramatic increase of the divorce rate suggests that mate selection is not as good as it was in the past, before oral contraceptives were widely used. Today, almost 50% of all first marriages in developed countries will end in divorce.

Second, there is an epidemic of depression -- Major Depressive Disorder -- in women which is easily four times greater than men. Much of this depression centers around relationships, or the lack of them, and feelings of loneliness, abandonment and of being unfulfilled in life.

Lastly, an unusually high percentage of children are being born with asthma, allergies, congenital deformities, autism and attention defecit disorders. Many of these problems can be traced back to homozygous offspring (i.e. a type of inbreeding).

While there is no hard evidence linking these problems with the use of oral hormones as a contraceptive, the logic is strong for an association.

How contraceptives work

Hormones are complex molecules that are controlled by a part of the brain called the pituitary. The interplay between pituitary and ovarian hormones, such as estrogen and progesterone, gives rise to a stereotyped cyclical pattern of hormone levels during the menstrual cycle. The graph below shows relative hormone levels in an average 28-day cycle.

The sequence of events in the menstrual cycle is determined by the relative hormone levels at each stage. About half way through the cycle, day 14, a viable egg will have been positioned in the reproductive system in such a way that it can be fertilized by a male's sperm. If this happens, certain hormonal changes will happen that allow the fertilized egg to grow and be nurtured. If the egg is not fertilized, different hormonal changes will happen that alter the reproductive tissue and result in menstruation.

The oral contraceptive pill alters the hormonal fluctuations associated with the menstrual cycle and essentially mimics the more steady hormonal conditions associated with pregnancy. Usually this is achieved by artificially introducing progesterone to the female's endocrine system, although some newer methods also introduce estrogen and progesterone.

Because progesterone prevents ovulation, the period of time immediately preceeding fertility, during which the female naturally selects a partner based on masculine traits and genetic diversity, is eliminated completely.

More important than eliminating the attractiveness to square-jawed, muscule men, scientists worry about the interference with a woman's instinctive attraction to genetically "opposite" partners. Altering this natural function could result in difficulties when trying to conceive, an increased risk of miscarriage and long intervals between pregnancies. Passing on a lack of diverse genes to a child could also weaken their immune system.

It's a matter of smell also

Hormones have a subtle, but powerful effect on our sense of smell. The best-known illustration of the invisible influence of scent is the way the menstrual cycles of women who live communally tend to synchronize over time.

How does one female signal the rest? The answer is almost certainly smell.

Pheromones -- or scent-signaling chemicals -- are known to exist among animals, and while scientists have had a hard time unraveling the pheromonal system in humans, they have isolated a few of the compounds. One type, known as driver pheromones, appears to affect the endocrine systems of others. Since the endocrine system plays a critical role in the timing of menstruation, there is at least a strong circumstantial case that the two are linked. "It's thought that there is a driver female who gives off something that changes the onset of menstruation in the other women," says chemist Charles Wysocki of the Monell Chemical Senses Center in Philadelphia.

Humans choose partners through their body odor and tend to be attracted to those with a dissimilar genetic make-up to themselves, maintaining genetic diversity. Genes in the Major Histocompatibility Complex (MHC), which helps build the proteins involved in the body's immune response, also play a prominent role in odor through interaction with skin bacteria. In this way these genes also help determine which individuals find us attractive.

As reported in ScienceDaily, a research team from the University of Liverpool (published in Proceedings of the Royal Society B: Biological Sciences) analysed how the contraceptive pill affects odor preferences. One hundred women were asked to indicate their preferences on six male body odor samples, drawn from 97 volunteer samples, before and after initiating contraceptive pill use.

Craig Roberts, a Lecturer in Evolutionary Psychology who carried out the work in collaboration with the University of Newcastle, said:

"The results showed that the preferences of women who began using the contraceptive pill shifted towards men with genetically similar odours."

"Not only could MHC-similarity in couples lead to fertility problems but it could ultimately lead to the breakdown of relationships when women stop using the contraceptive pill, as odour perception plays a significant role in maintaining attraction to partners."

The logic is simple. Find a partner with sufficiently different MHC, and you're likelier to carry a baby to term.

Choosing the wrong partner

A review of past research finds that, by altering hormonal cycles, the pill might affect choice of mates among members of both genders in a way that could hinder successful reproduction in the future.

"The use of the pill by women, by changing her mate preferences, might induce women to mate with otherwise less-preferred partners, which might have important consequences for mate choice and reproductive outcomes," said Alexandra Alvergne, lead author of a study appearing in the October issue of Trends in Ecology & Evolution.

"One prediction is that offspring of pill users are more homozygous than expected, possibly related to impaired immune function and decreased perceived health and attractiveness," according to the report by Alvergne, a postdoctoral research associate in the department of animal and plant sciences at the University of Sheffield in England.

Men's choices also altered

It's not just women who respond to such olfactory cues. Men, given a choice, will gravitate towards an ovulating female rather than a non-ovulating female.

One surprising study published last October in the journal Evolution and Human Behavior showed that strippers who are ovulating average $70 in tips per hour; those who are menstruating make $35; those who are not ovulating or menstruating make $50.

Other studies suggest that men can react in more romantic ways to olfactory signals. In work conducted by Martie Haselton, an associate professor of psychology at UCLA, women report that when they're ovulating, their partners are more loving and attentive and, significantly, more jealous of other men. "The men are picking up on something in their partner's behavior that tells them to do more mate-guarding," Haselton says.

Certainly more study is needed to determine the log term effects of oral contraceptives, not only on such things as cancer and heart related problems, but on the evolutionary changes resulting from yet another attempt to modify the natural order of things.